In the internal combustion engines of motor vehicles torque requirements are converted into mass flows. Each mass flow (mf) corresponds to an assigned injection time (ti) as a function of the fuel pressure (fup) (ti characteristic field). The relationship is used for all injectors, meaning that injector-individual differences, caused for example by manufacturing differences or ageing of the components over their entire lifetime, are not taken into consideration. Such differences between the actual mass flow and the required mass flow can cause effects such as making the mass flows too small (absence of injections, uneven running), making them too big (engine overheating) and making emissions worse.
There are currently two known processes which make it possible, at least partly, to adapt the ti characteristic fields of individual injectors.
a) IIC (Injector Individual Correction):
This method in which an individual correction of each injector is carried out was originally developed to increase the number of injectors yielded during production. In such cases, with a large number of injectors the ti characteristic fields are measured with mass flow measuring technology and an average ti characteristic field is calculated. The ti characteristic field deviations of all subsequently measured injectors in relation to the average ti characteristic field are measured for a few measuring points (for example for four measuring points MP1: 300 bar/4 mg, MP2: 700 bar/15 mg, MP3: 1000 bar/3 mg, MP4: 1600 bar/40 mg) and extrapolated on the basis of statistical methods for the entire ti characteristic field. The data is then stored for vehicle operation in corresponding characteristic fields.
With this method the measurement has to be undertaken on an injector test bed because of the measurement means needed. It is not possible to repeat the measurement while the vehicle is being driven. Although this method makes a correction of the ti characteristic field possible over the entire injection range, no correction of the values determined is possible during the lifetime (operating time) of the vehicle. This method is therefore restricted to the period before the vehicle is on the road.
b) MFMA (Minimum Fuel Mass Adaption):
With this MFMA method the deviations of the actual and required fuel mass in the minimum fuel mass range (<3 mg) are determined by means of engine speed changes and constantly adapted during the lifetime (operating life) of the motor vehicle. In such cases small injections are carried out at a cylinder in push phases in which no injections normally take place and the associated fuel mass is calculated based on models from a change in the engine speed (n). The correction values are stored in the characteristic fields for individual injectors for the tested minimum fuel masses.
This method is highly accurate and deviations are corrected during the lifetime. However it is only able to be used in the minimum fuel mass range (fuel masses<3 mg), since otherwise injections are perceived acoustically or as vehicle acceleration. Furthermore an expansion of the fuel mass correction beyond the minimum fuel mass range is not possible since the ti characteristic field has different gradients and corrections for an injector can be both positive and negative.
A correction for the complete ti characteristic field during the overall lifetime of a motor vehicle is thus not possible with the IIC and MFMA methods described above.